1. Field of the Invention
The present invention relates to rotary mechanisms in which one member, such as a shaft, is rotated relative to another member, such as a surrounding casing. For example, in actuators for driving various mechanisms such as valves, actuator assemblies may be provided which are operated by rotating an input shaft and which serve to convert this input rotation to rotary motion of the valve element about another axis or to linear motion. Such valves and their actuators are frequently buried underground, as for example in pipelines, or otherwise not susceptible to visual inspection. In such instances, the actuator input shaft or an extension thereof extends to the surface or to a point where operation of the actuator can be carried out.
The mechanism is usually designed to have a high mechanical advantage and low efficiency; thus a substantial number of turns of the shaft are necessary to bring the valve from a full open to a full closed position. This prevents the valve from being opened or closed too rapidly and renders the mechanism more capable of generating high output loads. However, the high mechanical advantage also cooperates with the high torque which may be generated in operation by the relatively long shaft to present a problem of excessively high input forces which can damage the actuator mechanism. This problem is further complicated by the fact that the operator of the mechanism can not see the valve or actuator, and thus it is difficult for him to tell if he has rotated the shaft too much.
One solution to this problem is to oversize the actuator so that it will be capable of withstanding these forces. However, this makes the actuator unduly expensive and bulky. The other alternative is to provide a stop means which prevents further rotation of the input shaft in one direction when the valve is open and preferably also prevents further rotation in the other direction when the valve is closed. This not only prevents the actuator mechanism from being damaged but also provides a means for the operator to know when the shaft has been rotated sufficiently to bring the valve to the desired position.
2. Description of Prior Art
One of the ways of providing stop means for the above purpose is to provide an elongate annular space between the input shaft, or its extension, and a stationary casing through which it extends. Opposed stationary stop surfaces are disposed at opposite ends of the space facing axially inwardly. A stop nut is disposed in the space coaxial therewith, i.e. surrounding the shaft. The nut is splined or otherwise keyed to either the shaft or the casing and is threaded to the other. Thus the nut can move longitudinally with respect to both the shaft and the casing, but rotates relative to only one of them. As the shaft is rotated, the nut will move along the annular space until it engages one of the stop surfaces whereby further rotation of the shaft will be prevented.
One of the problems with this type of stop assembly is that the same high forces which would cause damage to the actuator if the stop means were not present now cause jamming of the nut against the stationary stop surface so that extremely high separation forces are required to back the nut off from that surface when the valve position is to be changed.
This problem may be alleviated by providing a coarse pitch (more efficient) thread on the stop nut. However, this increases the overall length of the device and may cause the stop assembly to become overly bulky.